The most common type of hearing aid is one equipped with a so-called electrical volume, which is a means for converting input sound into electrical signal, then automatically correcting the degree of amplification of this electrical signal by digital processing, and thereby adjusting the volume of the output signal. On the other hand, many hearing aid wearers require a mechanical volume, with which a wearer can easily adjust the volume manually. Accordingly, there have been hearing aids equipped with both a mechanical volume and an electrical volume.
FIG. 5A shows an example of the configuration of a conventional hearing aid 1 (see Patent Citation 1, for example). The hearing aid 1 comprises a microphone 11, an amplifier 12, a receiver 13, a mechanical volume 14, and an electrical volume 15. With the hearing aid shown in FIG. 5A, the microphone 11 collects audible sound and converts it into an electrical signal, that is, an audible signal. The amplifier 12 amplifies the audible output signal from the microphone 11. The receiver 13 converts the audible output signal from the amplifier 12 into audible sound.
The mechanical volume 14 is constituted by a microminiature rotating variable resistor, is a mechanical volume adjusting means connected between the amplifier 12 and ground, and sets the upper limit for the level of an audible signal passing through the amplifier 12 to be adjusted.
The electrical volume 15 is also connected between the amplifier 12 and ground. The electrical volume 15 adjusts the level of the audible signal in tiny steps, and in turn performs volume adjustment, within the range of up to the upper limit set by the mechanical volume 14.
FIG. 5B is a diagrammatic perspective view of the above-mentioned hearing aid 1.
In FIG. 5B, a casing 16 has an insertion component 16a and a control component 16b. The insertion component 16a is the portion that is inserted into the ear, and the control component 16b is the portion the user operates to adjust the volume. The mechanical volume 14, the electrical volume 15, and the microphone 11 are attached to the attachment face 16c. The amplifier 12, the receiver 13, etc., are held inside the casing 16.
The operation of the above-mentioned conventional hearing aid 1 will now be described.
With the above-mentioned hearing aid 1, audible sound collected by the microphone 11 is converted into an audible signal and inputted to the amplifier 12. After the audible signal has been amplified by the amplifier 12, it is outputted to the receiver 13 and converted into audible sound. The hearing aid wearer sets the upper limit of the volume adjustment range by operating the mechanical volume 14, which makes use of the microminiature rotating variable resistor. The electrical volume 15 fine tunes the level of the audible signal, and in turn the volume, passing through the amplifier 12, in tiny steps, within the range of up to the upper limit thus set.
The volume of the hearing aid 1 is controlled by a volume switch (that is, the mechanical volume 14, which includes the variable resistor) and an LSI circuit connected to the volume switch (including the electrical volume 15 and the amplifier 12). For example, Table 1 shows an example in which the largest graduation on the volume switch is “5,” and the maximum resistance value of the variable resistor is 200 ohms, and the LSI chip subjects the resistance value to A/D conversion with eight bits. In this example, the relation between the value of the degree of amplification performed by the LSI chip and the volume is as shown in Table 1.
TABLE 1Degree of Value of Amplification byGraduation onVariableLSI (after Volume SwitchResistorA/D conversion)Volume00005 (MAX)200 ohms255Maximum volume (the upperlimit or less)
Specifically, when the graduation on the volume switch is at “0,” the value of the variable resistor (the resistance value) is zero ohms. When the LSI chip subjects this resistance value to A/D conversion with eight bits, the value is also “0,” and the volume outputted from the receiver 13 is “0.”
Meanwhile, if the graduation on the volume switch is at “5 (MAX),” the resistance value is 200 ohms, for example. In this case, the value of the degree of amplification by the LSI chip after A/D conversion is “255,” which is the maximum value produced by 8-bit A/D conversion, and the sound is reproduced at the highest volume (not greater than the upper limit).
FIG. 6 shows an example of the internal configuration of another conventional hearing aid which performs automatic control so that excessive volume is not outputted in the event that an impact noise or excessively loud noise is inputted (see Patent Citation 2, for example).
In the example shown in FIG. 6, when an impulsive input signal such as an impact noise or excessively loud noise is inputted to the microphone 10, which is an input transducer, this input signal is applied to the amplifier 30 via the capacitor C1 and amplified at a specific gain. The amplified signal is outputted by the earphone 20, which is a receiver equipped with a class D amplifier, via the capacitor C2.
DC power is supplied by the battery 80 (and the capacitor C3) to the microphone 10, the amplifier 30, and the earphone 20. The output signal (at connection point B) of the amplifier 30 is monitored by the amplification circuit 70 and the rectification circuit 50, and the input signal (at connection point A) of the amplifier 30 is attenuated by the biasing circuit 60 and the attenuation circuit 40 so as not to become excessively large.
The operation of the hearing aid thus constituted will now be described.
First, an input sound to the microphone 10 is applied to the amplifier 30 and the transistor 41.
The output from the amplifier 30 is applied to the earphone 20 and the amplification circuit 70. The AC signal applied to the amplification circuit 70 is rectified by diodes 51 and 52 in the rectification circuit 50, and is smoothed and converted into DC voltage by the smoothing capacitor 53. This DC voltage is added to bias voltage applied by the transistor 61 in the biasing circuit 60, and becomes the base voltage of the transistor 41 of the attenuation circuit 40. The transistor 41 here operates when the base voltage of the transistor 41 exceeds a threshold determined by the variable resistor 42, the signal from the input stage of the amplifier 30 is pulled in and attenuated, and the output of the earphone 20 is also suppressed.
Thus, no RC filter having a time constant or the like is used for the signal line inside the automatic gain control circuit, so response time from signal input to the start of suppression is short. When the input signal is small, the base voltage of the transistor 41 does not exceed the threshold determined by the variable resistor 42, so the input signal of the amplifier 30 is not attenuated, and there is no effect on the output of the earphone 20.